A network system is known in which transfer processing of a packet by a switch and control processing of the switch are separated. In recent years, attention is drawn to an open flow network system which uses an open flow protocol, as an example of such a network system. The open flow network system is configured from an open flow controller and open flow switches. The open flow controller and each of the open flow switches are connected by a link of switch control which is called a secure channel. Thus, a transfer control of the packet can be carried out by the open flow controller and the open flow switches. The open flow controller sets a flow entry to the open flow switch and the open flow switch transfers the packet based on the flow entry.
In the open flow network, when the load of the open flow switch is heavy, there is a case that the open flow switch cannot respond immediately to a control request transmitted from the open flow controller to the open flow switch. In such a situation, the flow entry cannot be set to the open flow switch, so that a packet transfer delay and a packet loss occur.
A technique is demanded by which the situation that the packet cannot be transferred in the network system can be avoided, even if the remaining switches operate normally because the load of a specific switch becomes heavier.
Patent Literature 1 (JP 2011-151718A) and Patent Literature 2 (JP 2011-166384A) are known as references in the field of the present invention.
The Patent Literature 1 discloses a network system which prevents the packet loss through the change of a packet transfer route. The network system of Patent Literature 1 is composed of a route management server and a buffering unit. In the network system of Patent Literature 1, the route management server sets a transfer route of the packet destined to a second node from a first node. When the route management server changes the transfer route into a new transfer route, a transfer route is set which passes through the buffering unit. The buffering unit sends out the packet according to an instruction from the route management server. Thus, the packet loss due to the change of the packet transfer route is prevented.
Patent Literature 2 discloses a technique for improving the fault endurance of a computer system which uses an open flow technique. The computer system in Patent Literature 2 is composed of a plurality of controllers and a switch. A priority is assigned to each of the controllers. The controller sets a flow entry with a priority to the switch. The switch determines the permission or non-permission of the setting of the flow entry according to the priority, and carries out a relay operation according to the flow entry set to the switch. Thus, the flow entry can be set to the switch without any contradiction even if the plurality of controllers exist in the computer system. The computer system of Patent Literature 2 improves the fault endurance of the computer system by multiplexing controllers.